Have you ever wondered why tides are lowest in winter along the northern Gulf of Mexico? Along our beaches, sand flats become exposed hundreds of feet from shore. In shallow bayous, the water can drain almost completely. Wonder no more, it is all about how our planet tilts and spins through the seasons, and the unique nature of the gulf we call home.
As a fisherman in coastal Louisiana, I always checked the tide charts to plan fishing trips into the local bays and bayous. In winter, launching a boat at some boat launches could be tricky, because of lower tidal range, and the effects of storm fronts. Fisherman and sportsmen knew the patterns. As an estuarine scientist, I learned why.
For one thing, the Gulf of Mexico (Gulf) is unique in that it only has one tidal cycle per day (one high and one low), unlike the Atlantic and Pacific oceans that have two. But the Gulf is a large, semi enclosed basin connected to the Atlantic Ocean through relatively narrow channels on either side of Cuba.
This geography restricts the rate of exchange of water between the Atlantic and Gulf over the daily tidal cycles, leaving us with only one cycle, sometimes two. There is not enough time between the two cycles on the Atlantic side to allow for full exchange with the Gulf. Circulation within this basin also has an effect on tides, with some of the lowest ranges in the western region of the basin.
But the number of tidal cycles per day (a high and a low) is only one characteristic of tides. Tidal range is the difference between how high tides rise and how low they retreat during any given cycle. This range changes during the regular 28-day lunar tidal cycle (the period in which the moon circles the earth) and during seasons of the year. Any tidal chart will show 2, 14-day sub-cycles for any lunar month that include spring tides (the highest, over a 5 to 7-day period) and neap tides (the lowest during a similar number of days).
What changes through the year (seasons) is the tidal range and the timing of high and low tides. The highest tidal range occurs in summer (about 4 ft during spring tides for the northern Gulf) with high tides occurring during daylight hours. The lowest range occurs in winter (1-2 feet for spring tides) with nighttime high ties. The ranges for neap tides are smaller but follow a similar pattern. This is largely driven by the tilt of the globe and the strength of the gravitational pull from the moon (by far the strongest pull on ocean water) and the sun (much weaker). In summer, our hemisphere tilts closer to both.
But weather can add another twist to the equation, especially in winter when the tidal range is lowest. Strong south winds ahead of a cold front can drive tides higher than expected. The shift to north winds after the front passes have the opposite effect, often retarding neap tides when water may not return at all.
So that is why tides are lower in winter in our part of the world. There are more details about how the moon and sun work to drive the tidal cycle, but the seasonal patterns are the most noticeable.
Hope to see you in our great outdoors!
Tide chart from Tide-forcast.com
